Here is one answer.
"Perko 0162DP0WHT Fog Horn Aluminum White"
We have kept one of these aboard our boat(s) since the 80's. Works really well, as long as you can draw a deep breath.

I have been told that from a distance it sounds like a bit like a love-sick goose! (Local humor relating to our goose population around the marina.)
Anyhow it's pretty loud and never needs a new canister like the old "air horn" we used to have... which would run out of compressed air at inopportune moments.
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